1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to a device with ultra-high capacitance and a method of forming such a device on a substrate.
2. Discussion of Related Art
A capacitor is a passive device that includes a dielectric layer sandwiched between two conducting layers which serve as the bottom electrode and the top electrode. The dielectric layer may be formed from Silicon Oxide (SiO2) with a dielectric constant, k, value of about 4.5. When a capacitor is formed on a substrate that includes other devices fabricated with silicon processing, both the bottom electrode and the top electrode may be formed from polysilicon. The resulting structure on the substrate is called a silicon-insulator-silicon (SIS) capacitor.
In some cases, the top electrode of the capacitor may be formed from metal while the bottom electrode of the capacitor may be formed from polysilicon. The resulting structure on the substrate is called a metal-insulator-silicon (MIS) capacitor.
In other cases, both electrodes of the capacitor may be formed from metal. The resulting structure on the substrate is called a metal-insulator-metal (MIM) capacitor.
The space occupied on the substrate by the capacitor may have to be reduced when other devices that are fabricated with advanced design rules are also located on the same substrate. Then, the capacitance density must be increased in order to maintain the same capacitance. Capacitance density may be increased by reducing the thickness of the dielectric layer. However, tunneling current may become excessive if the dielectric layer is too thin.
Capacitance density may also be increased by selecting a material with a higher k value for the dielectric layer. However, when the capacitor is fabricated on the substrate with other devices, the high-k material used for the dielectric layer may not be compatible with the processing of the other devices. For example, when the processing temperature is elevated, diffusion and reaction may affect the stoichiometry or phase of the high-k material.
Capacitance density may also be increased by increasing the surface area at an interface between the dielectric layer and the electrode. The surface area at the interface may be increased by roughening the surface of the electrode. When an electrode is formed from polysilicon, the surface of the electrode may be increased by forming larger grains in the polysilicon. Such films are called hemispherical silicon grain (HSG) or rough surface polysilicon (RSP). However, the grains may continue to grow during thermal processing of the substrate after formation of the dielectric layer, thus changing the resistivity of the electrode.
Thus, what is needed is a device with ultra-high capacitance and a method of forming such a device on a substrate.